Britain's soaring prison population is being driven by government policies rather than crime levels, according to an internal Whitehall briefing document seen by the Observer

The paper, a presentation to ministers and civil servants drawn up by the Ministry of Justice, explains in detail how new measures introduced by Labour have driven the prison population to record levels.

The admission is likely to embarrass the government as it suggests the prisons crisis - which has forced the prison service to use court and police cells to house offenders - is largely one of ministers' own making.

The paper acknowledges that successes in curbing reoffending rates, which have taken some pressure off prisons, have been all but wiped out by changes to the justice system. And it suggests further problems lie ahead with Britain's prisons running out of cells, possibly as early as the new year.

Britain's prison population stands at more than 83,000, just off its record high and close to full capacity. According to a graph in the briefing, the worst-case scenario could see it touch 86,000 early next year, suggesting there will not be enough room even if Operation Safeguard, the use of police cells, is reintroduced. The paper warns: "Capacity is likely to remain tight over the next 12 months and there is a possibility that Safeguard will be required again."

The paper states: "Prison population increases are driven by changes to the criminal justice system more than changes in offender behaviour." It acknowledges that this is the result of more and longer custodial sentences being handed down, more offences involving violence and drugs, and a greater use of recall to prison for prisoners who breach stringent rules governing their early release.

The admission that more people are being jailed at a time when crime is falling is likely to focus attention on the government's overhaul of the justice system over the past decade.

During the last 10 years the government has introduced 55 criminal justice bills, creating over 3,000 new criminal offences - which experts say have fuelled the increased use of custody - and eliminated its successes in reducing reoffending.

"This document makes it absolutely clear that the government's obsession with criminal justice bills, and the creation of more and more offences, is the prime cause of the rises in the prison population," said Harry Fletcher, assistant general secretary of Napo, the probation officers' union. "At the same time, this has been coupled with more severe sentencing. The situation is so grave that significant reductions in reoffending, achieved by staff working in probation and prisons, have been negated by the changes to the criminal justice system."

Alleviating the crisis will be difficult, as the paper makes clear criminal behaviour is closely associated with entrenched socio-economic factors. It says that "82% of offenders ... are at or below the writing level expected of an 11-year-old". And, it adds, "around two-thirds of prisoners who do have a job" lose it while in custody.